PSI - Issue 3

V. Di Cocco et al. / Procedia Structural Integrity 3 (2017) 217–223 Author name / StructuralIntegrity Procedia 00 (2017) 000–000

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Nomenclature LVDT Linear Variable Differential Transformer SEM Scanning Electron Microscope XRD X Ray Diffraction

Different shape memory alloys have been optimized in the last decades, such as the copper-zinc-aluminum (ZnCuAl), copper-aluminum-nickel (CuAlNi), nickel-manganese-gallium (NiMnGa), nickel-titanium (NiTi), and other SMAs obtained alloying zinc, copper, gold, iron, etc.. However, the near equiatomic NiTi binary system shows the most interesting properties and it is currently used in an increasing number of applications in many fields of engineering, for the realization of smart sensors and actuators, joining devices, hydraulic and pneumatic valves, release/separation systems, consumer applications and commercial gadgets as Otsuka et all. (2005) and Dong. et all. (2008). Due to their good biocompatibility, another important field of SMA application is in medicine, where the pseudo-elasticity is mainly exploited for the realization of several components such as cardiovascular stent, embolic protection filters, orthopedic components, orthodontic wires, micro surgical and endoscopic devices (Chen et all. 2005). From the microstructural point of view, shape memory and pseudo-elastic effects are due to a reversible solid state microstructural transition from austenite to martensite, which can be activated by mechanical and/or thermal loads as Liu et all. (2000). Copper-based shape-memory alloys are very sensitive to thermal effects, and it is possible that duringthermal cycles their properties change (e.g., shape-recovery ratio, transformation temperatures, crystal structures, hysteresis and mechanical behavior). Cu–Zn–Al shape memory alloys exhibit shape memory behavior in a range of composition. It is characterized by a stable high temperature and by a disordered bcc structure named β-phase. After a customized cooling process, a B2 structure is obtained, following a DO3 ordering. It is also know that martensite stabilization can be reduced by a step-quenched treatment. Cu ZnAl alloys mechanical properties are influenced by (Ameodo et all. 2003):  martensite stabilization;  grain size;  processes procedure (e.g., temperature, heat treatment cycles number). Other investigations carried out on CuZnAl alloys, showed the deformation influence on the macroscopic behavior and on martensite morphology. Martensitic transformation occurs initially in deformed material and the manufact shape follows the transformation as Kayali et all. (2000). Larger grains dimensions allow an easier transformation process, allowing the growth of 18R martensite (Zhang et all. 1999). In this work, damaging micromechanism during a tensile test in a CuZnAl alloy has been investigated, focusing the crack initiation and its stable growth. Deformation influence on alloy microstructure has been investigated during the tensile test by means of a X-Ray diffraction . 2. Material and methods In this work a CuZnAl pseudo-elastic alloy, made in laboratory by using controlled atmosphere furnace and characterized by chemical composition shown in table 1, has been used to investigate mechanical behavior in tensile conditions. The evolution of the microstructure during uniaxial deformation was analyzed by a miniature testing machine which allows in-situ scanning electron microscopic (SEM) observations as well as X-Ray micro-diffraction analyses. In particular, the testing machine is equipped with a simple and removable loading frame, which allows SEM and